1. Field of the Invention
The present invention relates to an image process apparatus for processing a color image signal and an image pickup apparatus having an image process function, and more particularly, to an image process apparatus and an image pickup apparatus adapted for processing a color image signal picked up by an XY address scanning type solid-state image pickup device.
2. Description of Related Art
Recently, image pickup apparatuses for picking up and storing an image using a solid-state image pickup device such as a digital still camera or a digital video camera come into wide use. As an image pickup device used for the image pickup apparatus, a charge coupled device (CCD) image sensor is most generally used. Recently, as the number of the pixels of the image pickup device has increased, a complementary metal oxide semiconductor (CMOS) image sensor is attracting much attention. The CMOS image sensor has a function of randomly accessing an image signal and has higher read-out speed, higher sensitivity, lower power consumption, compared with the CCD image sensor.
In addition, most of the image pickup apparatus has an automatic white balance function for automatically determining a white balance control value depending on a photographing light source. The photographing light source is generally estimated based on a pickup signal. As a general estimating method, there is a method of estimating the photographing light source from a sampling average of a high brightness portion or a middle brightness portion based on assumption called gray world in which a spectral reflection factor of a subject included in a photographed scene is averagely close to a flat. However, since this assumption may not be established in any subject, an adequate control result may not be obtained.
For example, when an RGB-color image pickup device having spectral sensitivity having a relatively high green response under a fluorescent lamp is used, a ratio of a gain of G to R and B under the fluorescent lamp is preferably lower than that of natural light. However, when the light source is not the fluorescent lamp but the color of the subject has a large amount of green component, the light reflected from the subject has a large amount of green component. Accordingly, it is difficult to estimate whether the light source is the fluorescent lamp or not and to obtain adequate white balance. At this time, when it is previously determined whether the light source is the fluorescent lamp or not, it is possible to improve the white balance control result in consideration of the characteristics of the image pickup device. Accordingly, there is a need for automatically and accurately determining whether the light source is the fluorescent lamp or not.
In addition, another method of estimating a photographing light source, there is disclosed a method of previously setting a plurality of light sources, comparing and detecting validities of the respective light sources at the time of control to perform estimation, and selecting and interpolating a control parameter suitable for the photographing light source from a preset value (for example, see Patent Document 1). In such a method, when the fluorescent lamp is not included in the set light sources, adequate white balance is not obtained under the fluorescent lamp, and, although the fluorescent lamp is included in the set light sources, it may be wrongly determined that the light source is the fluorescent lamp at the time of comparing and detecting.
In addition, in the image pickup apparatus, the pick-up color signal may be subjected to a correction process such as matrix conversion such that adequate color reproduction is observed in a display apparatus. A correction value of a matrix coefficient in this process varies by the photographing light source. In particular, in a light source having a color rendering property different from that of natural light, such as the fluorescent lamp, precision of color reproduction may not be sufficiently obtained in the same parameter as that of the natural light. Accordingly, even in the control of the color reproduction parameter, it is important to accurately determine the fluorescent lamp.
In the related art of determining whether the light source is the fluorescent lamp using the pickup signal, an external sensor is formed or a color filter having a band pass property is formed in an image pickup device or an optical system in order to determine whether the light source is the fluorescent lamp based on the spectral distribution characteristics. Accordingly, the manufacturing cost or the size of the apparatus increases. Meanwhile, a technology of determining whether a photographing light source is a fluorescent lamp based on a flicker detecting result using a method of reducing flicker of a pickup image generated by blinking of the fluorescent lamp is being considered.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-165932 (paragraphs [0034] to [0073] and FIG. 4)